Implantation is initiated when the blastocyst attaches to the uterine luminal epithelium and subsequently penetrates into the underlying stroma to firmly embed into the endometrium. While many aspects of implantation remain unclear, emerging evidence indicates that intercellular communication between endometrial epithelial and stromal cells is vital for successful establishment of pregnancy. The paracrine signals that mediate this crosstalk remain poorly understood. Our recent studies revealed that the transcription factor, hypoxia-?inducible factor 2 alpha (HIF2?), is induced selectively in endometrial stromal cells subjacent to the luminal epithelium at the time of implantation. This finding is significant because it is known for a long time that the maternal environment during implantation is hypoxic. To address the mechanisms of maternal adaptation to hypoxia during implantation, we generated Hif2?d/d mice that conditionally lack HIF2? in the endometrium. In these mice, the blastocysts are closely apposed to the uterine epithelium but fail to sustain firm attachment to it, resulting in implantation failure. The uterine stromal cells of Hif2?d/d mice exhibited downregulation of metabolic factors, such as lactate and a subset of amino acids, and a distinct set of Rab GTPases that regulate secretion of extracellular vesicles (EVs), including microvesicles and exosomes. The EVs are known to mediate a novel mechanism of cell-cell communication. That hypoxia promotes EV secretion by certain cells support the concept that HIF2?-driven EV secretion by the endometrial stromal cells represents a critical adaptive response that promotes intercellular communication during implantation. In the proposed study, we will employ the Hif2?d/d mouse model to test the hypothesis that HIF2? regulates a novel paracrine signaling mechanism under the hypoxic conditions of early pregnancy by controlling EV trafficking in the maternal tissue and that it directs transfer of key signaling molecules and metabolites from the stromal to luminal epithelial cells to influence epithelial functionality during implantation. We have proposed three specific aims to test this hypothesis. In Aim 1, we will investigate HIF2?-mediated EV trafficking in endometrial stromal cells and identify the resident molecular cargo in EVs. In Aim 2, we will investigate the effects of EVs and metabolic factors secreted from endometrial stromal cells on the functionality of epithelial cells. In Aim 3, we will investigate the role of HIF2? in regulating the secretory function of human endometrial stromal cells. A multi-pronged approach, utilizing a combination of (i) a unique animal model harboring a specific defect in adaptive response (ii) functional analyses using mouse and human primary endometrial cells, and (iii) genomic and proteomic analyses to understand cell-to-cell trafficking mechanisms, will enable us to provide answers to important unresolved questions regarding the impact of hypoxia on stromal-epithelial communication controlling endometrial function during early pregnancy.